U.S. patent application number 10/170770 was filed with the patent office on 2003-12-18 for transmit pre-equalization in a modem environment.
This patent application is currently assigned to ADC Broadband Access Systems, Inc.. Invention is credited to Unger, J. David.
Application Number | 20030231716 10/170770 |
Document ID | / |
Family ID | 29732584 |
Filed Date | 2003-12-18 |
United States Patent
Application |
20030231716 |
Kind Code |
A1 |
Unger, J. David |
December 18, 2003 |
Transmit pre-equalization in a modem environment
Abstract
The signal distortion present in a transmitted signal is
measured and used to generate filter coefficients. The coefficients
are stored in memory. A modulator generates an intermediate
frequency signal. The modulator inputs the signal to a programmable
predistortion filter. The characteristics of the filter are altered
by the filter coefficients that have been programmed into the
filter. The resulting filtered signal is upconverted to a frequency
in a transmit frequency band.
Inventors: |
Unger, J. David; (Windham,
NH) |
Correspondence
Address: |
Fogg Slifer Polglaze Leffert & Jay, P.A.
P. O. Box 581009
Minneapolis
MN
55458-1009
US
|
Assignee: |
ADC Broadband Access Systems,
Inc.
|
Family ID: |
29732584 |
Appl. No.: |
10/170770 |
Filed: |
June 13, 2002 |
Current U.S.
Class: |
375/296 ;
375/222; 375/298 |
Current CPC
Class: |
H04L 27/367
20130101 |
Class at
Publication: |
375/296 ;
375/298; 375/222 |
International
Class: |
H04K 001/02; H04L
025/03; H04L 025/49; H04L 027/36; H04L 005/16 |
Claims
What is claimed is:
1. A method for removing distortion in an information signal
generated in a cable modem termination system, the method
comprising: retrieving filter coefficients from memory; loading a
predistortion filter with the filter coefficients; and filtering
the information signal with the predistortion filter in response to
the filter coefficients.
2. The method of claim 1 and further including modulating the
information signal prior to filtering.
3. The method of claim 2 wherein the modulating comprises
quadrature amplitude modulating.
4. The method of claim 2 and further including upconverting the
filtered information signal prior to transmission over a cable
network.
5. The method of claim 1 wherein the information signal is a
baseband data stream originating from a Transmission Control
Protocol/Internet Protocol compliant network.
6. The method of claim 1 wherein the filter coefficients are
generated from an error between an ideal modulated information
signal and a distorted modulated information signal.
7. The method of claim 6 and further including: generating filter
coefficients in response to the error; and saving the filter
coefficients.
8. The method of claim 6 and further including: generating a
modulated data pattern with a modulation method in the cable modem
termination system; causing an activated equalizer to converge in
response to the modulated data pattern, the activated equalizer
having a substantially similar quantity of taps and substantially
similar sample rate as the predistortion filter; extracting real
and imaginary tap values from the activated equalizer after
convergence; and storing the real and imaginary tap values for use
as the filter coefficients.
9. The method of claim 6 and further including: generating a
continuous wave signal through a cable modem termination system
modulator; measuring a frequency response of the cable modem
termination system; determining an inverse frequency response in
response to the measured frequency response; and generating the
filter coefficients in response to an inverse Fast Fourier
Transform on the inverse frequency response.
10. A method for transmit pre-equalization of a modulated
information signal with a digital filter, the method comprising:
reading filter coefficients from memory; loading the digital filter
with the filter coefficients; and filtering the modulated
information signal with the digital filter by modifying the digital
filter's characteristics in response to the filter
coefficients.
11. The method of claim 10 wherein the modulated signal is a
quadrature amplitude modulated signal.
12. The method of claim 10 and further including: measuring
distortion in the modulated information signal without filtering;
generating the filter coefficients in response to the measured
distortion; and saving the filter coefficients in memory.
13. A method for transmit pre-equalization of an information signal
with a digital filter in a cable modem termination system, the
method comprising: modulating the information signal with a
quadrature amplitude modulation (QAM) technique such that a QAM
signal is generated; channel filtering the QAM signal; reading
filter coefficients from memory; loading the digital filter with
the filter coefficients; filtering the QAM signal with the digital
filter by altering the digital filter's characteristics in response
to the filter coefficients to generate a filtered signal;
converting the filtered signal from a digital signal to an analog
information signal; and upconverting the analog information signal
to a transmit frequency.
14. The method of claim 13 wherein upconverting comprises: mixing
the filtered signal with a first local oscillator signal to
generate a first mixed signal; and mixing the first mixed signal
with a second local oscillator signal to generate the information
signal at the transmit frequency.
15. The method of claim 14 wherein the transmit frequency falls in
a frequency band of 88 MHz to 860 MHz.
16. A method for transmit pre-equalization of a modulated
information signal in a cable modem termination system having a
predistortion filter, the method comprising: measuring a distortion
level of the modulated information signal; generating predistortion
filter coefficients in response to the measured distortion level;
and saving the predistortion filter coefficients in memory.
17. The method of claim 16 and further including: reading the
predistortion filter coefficients from memory; loading the
predistortion filter with the predistortion filter coefficients;
and filtering the modulated information signal with the
predistortion filter by altering the predistortion filter's
characteristics in response to the predistortion filter
coefficients.
18. A cable modem termination system comprising: a modulator for
generating a modulated information signal from a baseband signal; a
predistortion filter coupled to the modulator, the predistortion
filter generating a filtered signal in response to programmable
filter coefficients that have been generated in response to a
measured distortion of a signal transmitted from the cable modem
termination system; and an upconverter, coupled to the
predistortion filter, for mixing the filtered signal to a frequency
in a transmit frequency band.
19. The cable modem termination system of claim 18 and further
including a cable media access controller, coupled to the
modulator, that provides the baseband signal.
20. A cable modem termination system comprising: a cable media
access controller capable of providing a baseband data stream
signal from a network data signal having a first network protocol;
a quadrature amplitude modulation mapper, coupled to the cable
media access controller, for generating a modulated signal from the
baseband data stream signal; a channel filter that generates a
filtered signal from the modulated signal; memory that stores
programmable filter coefficients that have been generated in
response to a measured distortion of a signal transmitted from the
cable modem termination system; a digital predistortion filter
coupled to the channel filter, the digital predistortion filter
generating a predistortion filtered signal in response to the
programmable filter coefficients; an intermediate frequency
converter that generates a digital intermediate frequency signal
from the predistortion filtered signal; a digital to analog
converter that generates an analog intermediate frequency signal
from the digital intermediate frequency signal; and an upconverter
that converts the analog intermediate frequency signal to a signal
having a frequency in a transmit frequency band.
21. The cable modem termination system of claim 20 and further
including forward error correction
22. A method for transmit pre-equalization of an information signal
with a predistortion filter in a cable modem termination system,
the method comprising: modulating the information signal with a
quadrature amplitude modulation (QAM) technique such that a QAM
signal is generated; channel filtering the QAM signal to produce a
filtered signal; converting the filtered signal from a digital
signal to an analog intermediate frequency signal; reading filter
coefficients from memory; loading the predistortion filter with the
filter coefficients; filtering the filtered signal with the
predistortion filter by altering the predistortion filter's
characteristics in response to the filter coefficients to generate
an analog information signal; and upconverting the analog
information signal to a transmit frequency.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to modems and
particularly to data communication with a cable modem.
BACKGROUND
[0002] It is generally not practical in a communications system to
generate and work with frequencies that are the same as the
transmit frequency. Depending on the type of system, the transmit
frequencies may be greater than 100 MHz.
[0003] The generated signals, therefore, are mixed from an
intermediate frequency to a higher, transmit frequency by an
upconverter circuit. The upconverter may introduce noise and
frequency response distortion to the signal prior to
transmission.
[0004] Distortion may also be added to a signal prior to
transmission by any analog component that processes the signal. For
example, a digital to analog converter can introduce errors and
distortion.
[0005] A modulation error ratio (MER) is one metric used in the art
to indicate a signal's fidelity. The frequency response distortions
degrade the MER of the transmitted signal causing the receiver to
receive a signal that may be difficult to interpret or demodulate
properly.
[0006] The MER may be increased by manually adjusting components in
the analog circuitry or by using more accurate analog components.
Both approaches result in an increase in the expense of
manufacturing the electronic device. There is a resulting need in
the art to economically reduce frequency response distortions,
thereby increasing MER, prior to transmission of a signal.
SUMMARY OF THE INVENTION
[0007] The embodiments of the present invention encompass a method
for transmit pre-equalization of an information signal in a cable
modem environment. Filter coefficients that have been stored in
memory are read and loaded into a digital, predistortion filter.
The information signal is filtered by the predistortion filter. The
filter's characteristics are altered by the filter
coefficients.
[0008] In one embodiment, the filter coefficients are generated by
determining a difference between an ideal modulated information
signal and a distorted modulated information signal that is
transmitted from a cable modem termination system. The filter may
be implemented as either a digital or an analog filter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 shows a flowchart of one embodiment of a filter
coefficient generation method of the present invention.
[0010] FIG. 2 shows a block diagram of one embodiment of a
modulator and up-converter incorporating a predistortion filter of
the present invention.
[0011] FIG. 3 shows a flowchart of one embodiment of a transmit
pre-equalization method of the present invention.
[0012] FIG. 4 shows a block diagram of one embodiment of a cable
modem termination system in accordance with the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0013] The embodiments of the present invention provide a
programmable, predistortion filter in a communication device that
has modulation capabilities, such as a cable modem termination
system (CMTS). Programmed filter coefficients alter the filter's
characteristics to compensate for frequency response distortion,
thereby increasing the MER of a transmitted signal in a
communication system.
[0014] The subsequent discussion of the embodiments of the present
invention refers to the predistortion filter as it relates to a
cable modem system. Alternate embodiments use the predistortion
filter and methods in other types of communication systems that
implement some type of filtering operation to remove frequency
response distortion.
[0015] FIG. 1 illustrates a flowchart of one embodiment of a filter
coefficient generation method of the present invention. The
distortion generated by the analog circuitry of the CMTS is
measured (101) by determining a difference between an ideal
modulated information signal and a distorted modulated information
signal that is transmitted.
[0016] In one embodiment, this is accomplished by causing the CMTS
modulator to generate a data pattern with the desired modulation
(e.g., QAM64 at 5.12 Msymbols/sec.). The transmit output from the
CMTS is input to a vector signal analyzer with an activated
equalizer. In this embodiment, the equalizer is configured to have
a substantially similar number of taps and substantially similar
sample rate as the predistortion filter. Once the equalizer is
converged, the real and imaginary tap values are extracted from the
vector signal analyzer and used as the predistortion filter
coefficients (110) as described subsequently.
[0017] In an alternate embodiment, the filter coefficients are
generated by measuring the frequency response of the system by
forcing the modulator to generate a sweeping continuous wave (CW)
signal throughout the channel passband. The frequency response is
measured at the transmitter output using a spectrum analyzer or
power meter.
[0018] The inverse of the frequency response is then computed as
follows: if the measured frequency response is X(f), then the
inverse is computed by subtracting X(f) from the average of X(f)
over the channel passband. For example, if the passband is from
100-102 MHz, and if X(100 MHz) 2, X(101 MHz)=3, X(102 MHz)=4, then
the inverse would be 3-X(f). Note that smaller frequency steps
would normally be required for the desired accuracy. An inverse
Fast Fourier Transform is then performed on the inverse frequency
response vector to generate the predistortion filter coefficients
(110).
[0019] The above-described methods for generating the predistortion
filter coefficients are for illustration purposes only. The present
invention is not limited to any one filter coefficient generation
method.
[0020] The filter coefficients are stored in memory (115) for
future use. In one embodiment, the coefficients are stored in
non-volatile memory such as some type of programmable read only
memory. However, the present invention is not limited to any one
type of memory. The memory may be semiconductor, magnetic, or
optical.
[0021] The method illustrated in FIG. 1, in one embodiment, is
performed at approximately the same time that the electrical
circuit is fabricated. For example, after fabrication of an analog
circuit on a circuit board, the distortion caused by the circuit's
components is measured and the required filter coefficients are
stored in memory on the circuit board.
[0022] Each circuit board has a different set of components that
generates a different level of distortion. The filter coefficients
required to remove the distortion, therefore, are different for
each set of components.
[0023] FIG. 2 illustrates a block diagram of one embodiment of a
modulator and up-converter incorporating a predistortion filter
(203) of the present invention. In one embodiment, a digital data
stream signal at baseband frequency is input to the modulator
(201). This digital signal can be generated by any digital
circuitry and is not limited to the circuitry described herein.
[0024] In a CMTS embodiment, the modulator (201) performs the
quadrature amplitude modulation (QAM) required to transmit a
baseband signal from the CMTS to the cable modems on the subscriber
end. The baseband data stream is input to forward error correction
(210). The forward error correction (210) is a type of algorithm
used in one-way data communications where additional data is added
onto the digital signal to enable the receiving end to determine
that no errors are received.
[0025] The signal from the forward error correction (210) goes
through a QAM mapper (214). The QAM mapper (214) transforms the
data stream into a digital representation of a QAM modulated
signal. This occurs in both the real and imaginary domains.
[0026] The QAM mapped signal then goes through a channel filter
(211). The channel filter (211) is responsible for shaping the
spectrum of the signal to the desired shape, which minimizes
adjacent channel power and optimizes the system error rate (by
providing the signal shape for which the receiving device is
optimized).
[0027] The intermediate frequency signal from the modulator (201)
is then input to the digital predistortion filter (203) which adds
distortion to exactly counter (or cancel out) the distortion added
by all analog and digital signal processing between the QAM mapper
and the RF output of the CMTS. The filter (203) is loaded with the
filter coefficients that were generated in response to the
distortion caused by the analog or digital circuitry.
[0028] In one embodiment, these coefficients are read from memory
(206) where they were stored by the method of FIG. 1. The memory
may be incorporated into the filter, part of the same circuit card
as the modulator and upconverter, or it can be located in another
part of the system and coupled to the predistortion filter through
a data bus.
[0029] In this embodiment, the predistortion filter (203) is
illustrated as being in the digital domain. However, alternate
embodiments may implement the predistortion filter (203) as an
analog filter.
[0030] The predistortion filter (203) of the present invention is
not limited to any quantity of taps and/or sampling rate. These are
determined by the distortion characteristics for which the filter
is compensating and will be different for different situations.
[0031] The signal from the predistortion filter (203) is then mixed
in an I/Q mixer (213) with a signal from an oscillator (212) in
order to generate the intermediate frequency QAM signal. In the
CMTS embodiment, the oscillator generates a 44 MHz signal. The
present invention, however, is not limited to any one
frequency.
[0032] A digital to analog converter (215) transforms the digital
IF QAM signal to an analog information signal. This analog signal
is then input to an upconverter (205). The upconverter (205) is
responsible for generating a transmit frequency signal from the
intermediate frequency signal while reducing the occurrence of
spurious signals.
[0033] The upconverter (205) first mixes (217) a signal from a
first local oscillator (216) to produce a high frequency signal. In
a CMTS embodiment, this frequency is 1083 MHz. The high frequency
signal is then mixed (218) with a signal from a second local
oscillator (219) in order to generate the signal in the desired
frequency band for transmission. Mixing the signal first to a
higher frequency, and then down again keeps unwanted mixer products
out of the desired transmit band. In the CMTS embodiment, this band
is 88-860 MHz.
[0034] The frequencies illustrated in FIG. 2 are for purposes of
illustration only. Other embodiments use different frequencies to
accomplish substantially similar results. Additionally, alternate
embodiments mix the filtered signal with only one local oscillator
signal in order to upconvert the signal to the transmit band.
[0035] The use of the predistortion filter as shown in the CMTS
embodiment of FIG. 2 is for illustration purposes only. The
predistortion filter is not limited to only a CMTS embodiment but
may be incorporated in any electronic circuit that requires removal
of distortion using predetermined filter coefficients.
[0036] FIG. 3 illustrates a flowchart of one embodiment of a
transmit pre-equalization method in accordance with the embodiment
of FIG. 2. A baseband data stream is modulated (301) in order to
generate a QAM data signal.
[0037] The previously stored filter coefficients are read from
memory and loaded into the predistortion filter (305). The QAM
signal is then filtered with the predistortion filter using the
filter coefficients (310).
[0038] The filtered signal is upconverted from an intermediate
frequency to a transmit frequency (315). The upconverted signal can
then be transmitted (320).
[0039] FIG. 4 illustrates one embodiment of a CMTS incorporating a
predistortion filter of the present invention. The CMTS receives
data traffic coming from a group of subscribers and routes it to an
Internet Service Provider for connection to the Internet. The CMTS
also routes data from the Internet to the appropriate
subscriber.
[0040] The CMTS, in this embodiment, is coupled to an Ethernet
network that transmits data using Transmission Control
Protocol/Internet Protocol (TCP/IP). The TCP/IP data is coupled to
a cable media access controller (401). The media access controller
(401) acts as an interface between the hardware and software
portions of various network protocols. In one embodiment, the media
access controller (401) translates the TCP/IP into whatever
protocol is being used on the downstream (i.e., to the subscriber's
modem) side of the system.
[0041] In another embodiment, the media access controller is
responsible for loading the filter coefficients that are stored in
memory (403) into the predistortion filter (203). The embodiment of
FIG. 4 shows the memory (403) as being a separate block. One
embodiment incorporates the memory into the media access controller
(401). Another embodiment incorporates the memory into the
predistortion filter (403).
[0042] In the embodiment of FIG. 4, the predistortion filter (203)
is illustrated as being located after the modulator (201). This
location is outside the digital to analog converter in the
modulator (201). Therefore, the predistortion filter (203) is
filtering an analog signal as opposed to the digital signal of the
embodiment of FIG. 2. The location of the predistortion filter
(203) is not critical.
[0043] One embodiment uses a dedicated controller (e.g., POWERPC)
to perform the media access controller (401) function. If the CMTS
is part of a larger system (e.g., computer system), the media
access controller's functions may be offloaded or shared by the
system's controller.
[0044] The baseband data signal from the media access controller
(401) is input to the modulator (201). The QAM signal from the
modulator (201) is input to the predistortion filter (203) to add
pre-distortion. The filtered signal is then upconverted (205) to
the transmit frequency. In one embodiment, the signal at the
transmit frequency is coupled to the subscriber modems over
cable.
[0045] An upstream side of the system comes from the subscriber's
cable modems to the CMTS. The CMTS may have multiple receivers
(405-407) that demodulate the signals from the upstream connection.
Each receiver may be assigned to one subscriber modem or one
receiver may be assigned to groups of subscriber modems. The
demodulated signals from the receivers are input to the cable media
access controller (401) for conversion to the network protocol.
[0046] Numerous modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *